1. Start the computer. The screen is black. Use a flashlight, and shine it on the screen. Do you see anything of your desktop (icons, background picture, start menu, etc.)?
If you do, then the backlight of your screen seems to have some contact failure.
2. If you have a monitor that you can join to the laptop, see if that gives any picture.
If it does, then you'll know, that it is indeed the laptop's screen's error, and not some software error. (But I doubt it would be software error, since it's black from the beginning - you should see something from the boot process, you should be able to go into BIOS, etc.)